The JAK (Janus kinase) family is a tyrosine kinase family consisting of four members, JAK1, JAK2, JAK3 and Tyk2 (Tyrosine kinase 2) and plays an important role in cytokine signaling.
While the kinases of this family, except for JAK3, are widely expressed in tissues, expression of JAK3 is restricted to immune cells. This is consistent with the fact that JAK3 plays an important role in various receptor-mediated signaling pathways such as IL (interleukin)-2, IL-4, IL-7, IL-9, IL-15 and IL-21 signaling by noncovalently associating with the common γ chain (Non-Patent Documents 1 and 2).
Lowered JAK3 protein levels or defects in the common γ chain gene observed in patients with an immunodeficiency called X-linked Severe Combined Immuno Defficiency (XSCID) suggest that blocking of the JAK3 signaling pathway leads to immunosuppression (Non-Patent Documents 3 and 4). Animal experiments indicate the importance of JAK3 not only in maturation of B- and T-lymphocytes but also in maintenance of T-lymphocyte functions. Therefore, regulation of immune responses via this mechanism is a promising therapy for T-cell lymphoproliferative diseases such as organ transplant rejection and autoimmune diseases.
Analyses of JAK1 knockout mice and JAK1-deficient cells suggest involvement of JAK1 in various receptor-mediated signaling pathways such as IFN (Interferon)α, IFNβ, IFNγ, IL-2, IL-4, IL-6, IL-7 and IL-15 signaling (Non-Patent Document 5). Therefore, regulation of inflammatory responses via these signaling pathways is therapeutically promising for treatment of diseases involving macrophage and lymphocyte activation such as autoimmune diseases and acute and chronic organ transplant rejection.
Analyses of JAK2 knockout mice and JAK2-deficient cells suggest involvement of JAK2 in various receptor-mediated signaling pathways such as EPO (Erythropoietin) α, thrombopoietin, IFNγ, IL-3 and GM-CSF signaling (Non-Patent Documents 6, 7 and 8). These signaling pathways are supposed to mediate differentiation of erythrocyte or thrombocyte progenitor cells in bone marrow. Meanwhile, it is suggested that a substitution of phenylalanine-617 with valine in JAK2 is associated with myeloproliferative diseases (Non-Patent Document 6). Therefore, regulation of differentiation of myeloid progenitor cells via these signaling pathways is therapeutically promising for treatment of myeloproliferative diseases.
The JAK inhibitor CP-690,550 is reported to have improved the pathology of rheumatoid arthritis and psoriasis in clinical tests (Non-Patent Documents 9 and 10) and suppressed rejection in a monkey model of kidney transplantation and airway inflammation in a murine asthma model (Non-Patent Documents 11 and 12). From these findings, immunosuppression by JAK inhibitors is considered to be useful for prevention or treatment of organ transplant rejection and post-transplant graft-versus-host reaction, autoimmune diseases and allergic diseases. Although other compounds having JAK inhibitory action than CP-690,550 have been reported (Patent Documents 1 to 11), development of more of such compounds is demanded.